German Patent Application No. 38 06 237 describes a process for the preparation of N4-substituted 1-methoxy-2,4-diaminobenzenes by the reaction of 4-amino-2-nitro-anisole with chloroalkyl-chloroformates in different aqueous solvent mixtures containing a buffer. Reactions are carried out in a temperature range from about 70° C. to the reflux temperature of the solvents or solvent mixtures used; reaction times between about 4 and about 12 hours are required. The open chain carbamates are isolated by addition of water. Further steps are carried out in strong basic aqueous media, which is completely unfavorable in terms of the presence of 4-amino-2-nitro-anisole.
German Patent Application No. 35 36 066 describes a multi-step process for the preparation of N,N′-disubstituted p-phenylenediamines by reacting a 4-halogen-3-nitro-aniline with a β-chloroethyl-chloroformate and forming an oxazolidone by addition of an alkali metal hydroxide in an organic solvent, then condensing the oxazolidone with a primary amine and at the end alkaline hydrolysis of the reaction product.
Problems concerning 4-amino-2-nitro-anisole residues have also been described in U.S. Pat. No. 5,508,467, which relates to a process for the preparation of hydroxyalkylamino-nitro-benzenes. Although U.S. Pat. No. 5,508,467 teaches that highly pure products can be provided by this process, the amount on residual 4-amino-2-nitro-anisole, which is 480 ppm, is still unsatisfactory. The 4-amino-2-nitro-anisole cannot entirely be removed by multiple recrystallization without considerable loss in yield. If the target nitro compounds would be reduced into the corresponding amines, it would therefore be impossible to separate the resulting 2,4-diaminoanisole by-product from the target compounds.
So it is essential to find a process for the synthesis of 4-hydroxyalkylamino-2-nitro-anisoles which gives extremely low amounts of residual 4-amino-2-nitro-anisole, because to be used as intermediate for the synthesis of hair dye precursors 4-hydroxyalkylamino-2-nitro-anisoles must contain significantly lower amounts of 4-amino-2-nitro-anisole than feasible by the methods known in the state of the art.